Compounds · EduSite Academy

▶ Compounds and chemical formulas

Some substances, like copper or gold, are pure elements. Others, like water (H₂O), are compounds—made from two or more elements in fixed ratios, chemically bonded. Each compound has a unique formula, like CO₂ for carbon dioxide, representing its specific atomic arrangement. Compounds form molecules, tightly bonded units, creating an incredible diversity of substances in nature.

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▶ Molecules, crystals, and diatomic elements

Elements and compounds can exist as pure substances, each with a fixed, definite composition. Some compounds, like water, are made of individual molecules. Others, like salt, form crystals—large, repeating 3D patterns without individual molecules. Additionally, some elements pair up as diatomic molecules, such as O₂ and H₂, rather than existing as single atoms. Understanding whether a substance is molecular or crystalline and how diatomic elements differ from compounds is key to classifying matter.

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📄 Learn and try: Molecules, compounds, and chemical formulas

The words you are reading right now are made from just 26 letters combined in different ways. Each word uses a different combination of these same letters.

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Similarly, everything around us is made from about 100 elements combined in different ways. The atoms of each element are like nature's letters, and the words they make are called molecules.

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In this article, we'll explore how atoms bond together as molecules and ionic crystals, forming the compounds that make up our world.

##Molecules are groups of atoms that act as a single unit

Let's start with molecules. A molecule is defined as two or more atoms that are chemically bonded in a fixed ratio. The atoms are joined together and act as a single unit.

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This is different from just mixing elements together—when elements are chemically combined, they form something completely new with different properties.

A molecule can be composed of multiple atoms of the same element. For example, the oxygen we breathe consists of two oxygen atoms joined together. When an element naturally exists as a two-atom molecule like this, we call it a diatomic element.

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A molecule can also be a combination of different elements. For example, we release carbon dioxide when we exhale, which is a molecule composed of one carbon atom and two oxygen atoms.

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These molecules—whether made of the same or different elements—are all separate units of atoms bonded together in fixed amounts.

##Molecular and crystalline compounds

The word "molecule" describes the structure of the particles—atoms bonded together as a single unit. Carbon dioxide is a molecule. We can also use another term to specify that it is composed of two different elements.

A compound is a substance made of two or more different elements that are chemically combined in a fixed ratio. When compounds exist as individual molecules—like carbon dioxide or water—we call them molecular compounds.

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But some compounds do not exist as molecules. Crystalline compounds form when different elements arrange in a repeating pattern that extends throughout the entire substance. This creates one large, organized structure known as a crystal lattice.

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Notice there are no individual molecules in the crystal structure of lithium chloride above. And even though there are many atoms shown in this model, we can still see a consistent $1{:}1$ ratio of lithium atoms to chlorine atoms. Like molecular compounds, crystalline compounds also contain different elements combined in fixed ratios.

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##The same elements in the same proportions

Molecules and compounds always have the same elements combined in the same proportions. This is what is we mean by a fixed ratio.

For example, carbon monoxide is always made of one carbon atom and one oxygen atom. However, carbon dioxide is always made of one carbon atom and two oxygen atoms.

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Each substance has properties that are unique to its specific ratio of elements. Carbon monoxide is a poisonous gas while carbon dioxide creates the bubbles in soda—one additional atom makes a big difference!

###Chemical formulas show the ratios

Let’s look at how these ratios are represented in chemical formulas.

A chemical formula uses numbers and symbols from the periodic table to show the ratio elements in a substance. For example, carbon monoxide is represented with the formula $\ce{CO}$, which tells us it is made of one carbon atom $\ce{(C)}$ and one oxygen atom $\ce{(O)}$.

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Carbon dioxide is represented by the formula $\ce{CO2}$. This tells us it is made of one carbon atom and two oxygen atoms. A sample of carbon dioxide contains billions of molecules but they all look the same: one carbon atom bonded to two oxygen atoms.

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Notice the small $2$ in the formula $\ce{CO2}$. These little numbers are called subscripts. They are written to the right of and slightly $_{below}$ the element they refer to. When there is no subscript after a symbol, it means there is only one atom of that element.

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The examples given above describe molecular substances, but what about crystalline compounds? How can we possibly count all of the atoms in the sample of zinc bromide below?

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Recall that all compounds have fixed ratios. Although the model above has many atoms, there is a consistent $1{:}2$ ratio of zinc atoms $\ce{(Zn)}$ to bromine atoms $\ce{(Br)}$ throughout.

When we describe crystalline compounds in chemistry, we use the simplest whole number ratio of atoms in the compound. So, this compound would have a chemical formula of $\ce{ZnBr2}$.

##Elements and compounds are pure substances

We are now familiar with the building blocks of matter—elements and their atoms—and how they combine to form different types of substances like diatomic elements, molecular compounds, and crystalline compounds.

These are all examples of pure substances. A pure substance is defined as having the same composition throughout—all the particles within the sample are identical.

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For example, a sheet of pure copper, $\ce{Cu}$, is made entirely of copper atoms. A tank of nitrogen gas—a diatomic element that forms $\ce{N2}$ molecules—contains only nitrogen.

The same applies to compounds: a sample of pure carbon dioxide contains only $\ce{CO2}$ molecules. It does not contain separate $\ce{C}$ atoms or $\ce{O2}$ molecules, only billions of the same $\ce{CO2}$ molecules!

In crystalline compounds, the same pattern of particles repeats throughout the entire crystal. Sodium chloride, $\ce{NaCl}$, has one atom of sodium for every one atom of chlorine, which is consistent throughout the entire sample.

Notice that in each case, the composition never varies within the sample. However, when compounds are mixed with other compounds or elements, the sample is not considered pure and is classified as a mixture instead.

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##Key points

A molecule is a group of atoms chemically bonded together in a fixed ratio.
Diatomic elements are elements that exist as molecules in nature. Each molecule contains two atoms of the same element.
A compound is a substance made of two or more different elements that are chemically combined in a fixed ratio.
Crystalline compounds have a repeating pattern of different elements arranged in a crystal lattice, while molecular compounds exist as a single unit.
A chemical formula uses numbers and symbols from the periodic table to show the ratio elements in a substance. The small numbers within the formula are called subscripts.
All compounds and elements have the same composition throughout and are classified as pure substances.

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